Induction Agents

Induction Agents

• Each agent has its own mode of action

• All result in loss of consciousness

  • Unconsciousness is achieved once sufficient brain levels obtained

  • Movement of agent from blood to brain occurs via concentration gradients

  • Speed of induction and recovery determined by the lipid solubility of agent

  • Drugs that are highly lipid soluble cross cell membranes faster (faster onset and recovery)

Upon injection of induction agent

Upon injection of agent

• High levels of drug in blood

• These drugs rapidly are taken up by brain

• Low level in muscle, fat, and organs

Once injection is completed

• Levels in blood drop

• Agent leaves brain because of concentration gradient

• Agent leaves blood and enters tissues

Recovery occurs

• Once levels in brain become low enough, the animal recovers consciousness

• The drug however is still in the fat, muscle, and tissues

• Different drugs excreted differently, but patient not fully recovered until all agent has left body (may take >24 hrs)

Propofol

• Ultra-short acting

• Nonbarbiturate

• Used for induction and maintenance of anesthesia

• Also used for treatment of status epilepticus

• Non-Controlled — but many practices will still record in drug log as a precaution

Pharmacology

• Minimal water solubility

• Milky, but can be given IV

Onset of action: 30-60 sec Duration: 5-10 min

Propofol effects

Major effects

• Dose dependent CNS depression

• Cardiac depression — bradycardia, decreased CO, hypotension

• Respiratory depressant — APNEA

• Twitching during induction

• Muscle relaxation

Minor effects

• Relatively safe for animals with liver and kidney dysfunction

• Appetite stimulant at low doses

• Antiemetic

• Decreases intracranial and intraocular pressure

Adverse effects

• Excitement during induction if injection is too slow

• Hypotension — may be prolonged if patient previously hypotensive

• Induction apnea

• Irritating when given SQ

• Heinz body formation in repeated dosing to cats

Use

• Give dose slowly to effect (bolus first half, then give to effect)

• Can also be used as CRI to maintain GA

• Manufacturer recommends discarding product after 6 hrs of opening

• Cost is 4X that of ket/val

Etomidate

Duration of action: 3-5 min

Effects

• Minimal effects on cardio and resp systems

• Little analgesia

• Anticonvulsant

• Good muscle relaxation

• Pain on injection

• Nausea & vomiting

Not available in Canada

Alfaxalone

• Ultrashort-acting injectable anesthetic

• Wide margin of safety

• Relatively new in Canada

• Can be given IV or IM

• Works similar to propofol

Effects

• Dose-dependent CNS depression & resp depression

  • Apnea

• Hypotension (especially when used with inhalant)

• Muscle relaxation

• No analgesia

• Cardio system depression minimal

• Smooth induction, no excitement

• Not irritating if inadvertently given perivascular

Use

• Given similar to propofol — bolus ~half, then to effect

• Duration 5-10 min

• Repeated dose does not prolong recovery, therefore CRIs are possible

• Better shelf life

Dissociative Anesthetics

Drugs

• Ketamine

• Ketamine-Diazepam combo

Onset of action

• 1-2 min IV

• 10 min IM

Duration

• 20-30 min

• Higher dose increases duration, not depth

Use

• Can be given IM or IV

Dissociative Anesthetics effects

Effects

• Cataleptic state — patient does not respond to stimulus and maintains muscle rigidity

• Intact reflexes — difficult to assess depth

• Central eye that has mydriatic pupils and open lids

• Normal or increased muscle tone

• Analgesia to skin and limbs, limited to viscera

• Amnesia — humans

• Increased sensory stimuli

• Do not become bradycardic

Adverse effects

• Stimulation to sound, light, etc.

• Abnormal behaviors in recovery

• Nystagmus

• Decrease contractility — patient with heart disease cannot compensate

• Predispose to development of arrhythmias

• Increase salivation and resp secretions

• Tissue irritation if given perivascular

• Increased intracranial and intraocular pressure

Ketamine

• IM use is limited to fractious cats

• Can be given orally to fractious cats

• If given repeated doses, can increase risk of seizure activity and prolong recovery

• Undergoes hepatic metabolism and renal excretion — use caution if renal or hepatic disease

Ketamine-Diazepam

• IV induction for cats and dogs

• Equal volumes of each mixed in same syringe (watch for precipitation)

• Give to effect

• Duration: 5-10 min

• Minimal cardiac depression

• Good muscle relaxation

• Superior recovery

• Some analgesia

Guaifenesin (GGE)

• Not an anesthetic on its own — given in combination with other agents

• Needs to be reconstituted

Effects

• Muscle relaxation

• Minimal cardiopulmonary effects

Adverse effects

• Can cause thrombophlebitis or tissue reaction if given perivascular

Use

• Used in combination with other agents for induction and maintenance of anesthesia

• Given rapidly IV after premed until signs of knuckling noticed, then induction agent given